Omkring Radar Reflektor.

97 % af de Radar Reflektore der i dag sidder på de danske fritidsfartøjer har en meget ringe eller slet ingen virkning !
At vælge den bedste Radar Reflektor kan være ret frustrende og ikke en ligefrem opgave, da der er flere faktorer der spiller ind.
Vi har lavet en liste med nogle af de testresultater, vi selv har været igennem i vores søgen på en brugbar Radar Reflektor.
Vores valg er faldet på en Aktiv Radar Reflektor af mærket: SEA-ME samt en Pasiv af mærket: ECHOMAX.
Har du mod på at tygge dig igennem nedenstående tests og undersøgelser, bliver du noget klogere.
Har du mod på mere, så har vi lagt kilde henvisninger under det helt - GOD FORNØJELSE !

1995 Radar Reflector Test.

Almost nothing provokes as much fear in the hearts of sailors as the thought of a collision with a ship. Rogue waves and killer storms, maybe, but there are a lot more ships than either of those. The good news is that a ship keeps a careful watch and they always use radar, so avoiding a collision is simply a matter of a good radar reflector.

Or is it? Just how well do radar reflectors work, anyway?

Thanks to the generous cooperation of SRI International, in Menlo Park, CA, we had an opportunity to find the answer to that question. West Marine provided samples of 10 commercially available radar reflectors, which were tested in SRI’s large radar test chamber, normally used for testing such things as satellite antennas and stealth bombers.

Participating In the tests were Eldon Fernandes, the operator of the range and an employee of SRI; Dick Honey, a Sr. Principal Scientist at SRI; Stan Honey, Vice President of Technology for News Corporation and a former Research Engineer at SRI; Chuck Hawley, Technical Director for West Marine; and Jim Corenman, Sailor at Large.

Each reflector was mounted on a pedestal inside SRI's anechoic chamber. SRI's Eldon Fernandes, a development engineer for the Remote Measurements Laboratory, assists with the data logging.

Characteristics of Marine Radar:
Marine radar comes in two flavors, X-band and S-band. Ships will typically carry both, while small vessels are limited to the smaller X-band units. X-band radar operates at a frequency of approximately 9.4 GHz (9400 MHz), with a wavelength of 3.2 cm, while S-band operates at approximately 3 GHz, with a longer wavelength of 10 cm. X-band radar offers greater resolution and detection of smaller targets, but is more susceptible to interference from rain and seas (sea clutter). S-band radar has longer range and less interference from rain and sea clutter, but has less sensitivity for small targets.

A ship will typically use her X-band unit near shore, due to its higher resolution and ability to detect smaller targets. In conversations with ship’s officers, nearly all indicated that in offshore waters they depend entirely on the S-band unit set to a 24-mile scale. The advantage of S-band in this situation is longer range, less interference from rain, and reduced interference from sea clutter (a factor of about 2½, or -4 dB).

This is not good news for radar reflectors, however, since performance falls off as the square of wavelength. This means that, at least in theory, a given reflector will have an S-band return of only one tenth (-10 dB) compared to its X-band performance. In a situation where the return from the sea state is the limiting factor, part of this loss is made up by reduced sea clutter, but the effective return will still be reduced to one fourth (-6 dB) compared to X-band.

A digression on units of measurement … Radar reflector performance is normally characterized in terms of Radar Cross Section, or RCS, measured in square meters (m2). The measurement of RCS is referenced to a conductive (metal) sphere of the specified cross-sectional area, using the familiar p r2 formula for the area of its cross section. Performance of a reflector is expressed in decibels (dB) relative to some reference, typically a 1.0 m2 sphere, but occasionally some other reference. Decibels are a relative measurement, and are log-based, with 3 dB representing a factor of two, and 10 dB representing a factor of 10. So saying that a signal is 3 dB below a 1.0 m2 reference (or -3 dB) is the same as saying it is half as big, or 0.5 m2.

One thing that helps put everything into perspective is to consider that the radar return from a typical duck is about 0.1 m2, so our 1 m2 reference sphere can be also described as 10 duck units, or 10 du’s. Not very big. So having properly introduced the technically hip decibel units, we will now go on and talk about things we can visualize, like square meters and duck units. Everyone understands that, when it comes to radar reflectors, more ducks is better.

The central question, of course, is just how many ducks does it take to be seen by a ship? There are as many answers as there are ships and radar operators, but the typical response that keeps coming back is that the lower limit of detectability using X-band radar in a moderate sea is 1 to 3 m2 (10-30 du’s). GEC Marconi (the manufacturer of the popular Firdell Blipper), statesi that 2.5 m2 is "generally accepted in the radar business as the ‘threshold’ of radar detectability" at X-band, and proposes a minimum average RCS of 2.5 m2 (25 du’s), with no gaps below 2.5 m2 that are larger than 5° x 5°.

In response to the question of how far away a sailboat can be picked up on radar, the most common answer is three to six miles without a radar reflector if it can be picked up at all, and a better chance of picking it up with a reflector. These numbers are purely anecdotal, based on conversations with ship’s officers, but are consistent with those reported in Radar Detectability and Collision Riskii.

These detection ranges are certainly less than those of us who sail small vessels would hope for, and less than we’ve somehow been led to believe. What we will try to accomplish in the space that remains is to sort the truth from the claims, and bring some perspective to bear on the problem of visibility at sea.

The one thing that is key to the performance of any reflector design is size. The reflective performance of any type of reflector is proportional to the fourth power of its linear size. In other words, doubling the size of a reflector results in an increase of effective area of 16 times, or a 12 dB increase. Stated differently, an increase in size of a reflector of 19% will double its performance. Further, as the smallest dimension of a reflector gets down to a few wavelengths of the radar signal, it quits acting as a reflector and starts to act as a lump of metal. Remember that a wavelength is 3.2 cm (1¼") for X-band, and 10 cm (4") for S-band. So small detectors must be looked at with a great deal of suspicion, as there really is no substitute for size.

Another issue is the increasing reliance upon ARPA systems aboard ships. These systems automatically capture and track radar targets, and provide a warning to the watch when a close approach is predicted. An ARPA system will only work with targets that are visible on the radar, however, and typically a minimum of three consecutive "hits" is required on the ship’s radar before a blip is acquired as a target. This puts a premium not only on the strength of the return, but also its consistency.

The Reflectors Tested:
Most radar reflectors are variations on the 3-sided corner reflector, also known as a corner cube or a trihedral reflector. The principal echo from a trihedral reflector will be strongest when its "pocket" is oriented directly towards the radar. As the trihedral reflector is rotated off this axis in any direction, the echo becomes weaker, and drops by half (-3 dB) at an angle of 12° to 20° from the axis of symmetry, depending on its specific shape (see fig. 1). With increased rotation, the return continues to drop to almost zero as one of the three sides approaches an edge-on attitude to the radar. When one edge is exactly edge-on, there will be a strong but narrow return, caused by the other two edges acting as a dihedral (2-sided) reflector, or one side acting alone as a flat plate reflector. These returns can be very strong, but so narrow in angle as to have little value.

Octahedral Reflectors:

The classic octahedral reflector is made of three planar circles or squares of metal intersecting at right angles, forming eight trihedral reflectors. In the usual "catch rain" position, one trihedral will face up and one down, and the remaining six are arrayed around a circle, three oriented 18° above the equator, and three 18° below. This optimizes the return from the "pockets", and avoids the nulls or gaps as best as is possible, but only at a 0° angle of heel.

Considerations of heel angle has led to the "double catch rain" position (see fig. 2), with one planar surface oriented vertically along the vessel’s axis, and the other two planes ±45° from the vertical. This is not the ideal with no heel angle, but moves towards the "catch rain" position as the boat heels.

Davis Echomaster and Emergency:

The Davis Echomaster is available in standard and deluxe models. The deluxe has a mounting harness. Mounted in the double catch-rain position, it rated very well.
The Davis Emergency is made of foil laminated over foam. Square alignment of the plates is important to its effectiveness.

We tested three octahedral-type reflectors. Two Davis products were included, the 6.25" radius spherical Echomaster, model 153, and the Emergency model 151. The Emergency model has circular plates of 5.5" in radius, and is constructed out of foil laminated over a foam core. It can be disassembled into three disks for storage if desired. The Echomaster Deluxe is constructed from anodized aluminum disks and has a radius of 6.25". It comes with a plastic and stainless bracket which attaches to the intersection of all of the plates, although it was removed for our tests. When installed, this bracket makes is easier to suspend or mount the device in the so-called "catch rain" position.

Holland Yacht Equipment:

The HYE has no provisions for mounting other than small holes in the corners
The Holland Yacht Equipment (HYE) #1274 is an aluminum octahedral reflector with triangular pockets. This means that the plates from which it is made are square before assembly, and the plates intersect across the diagonal of each plate, thus forming the triangular pockets. It measured 4.7" in radius, and is constructed such that the slots in each plate assure alignment when assembled which is accurate to a few degrees. Other than small holes in each corner, the HYE has no provision for mounting.

Other Trihedral-based Reflectors

The Firdell Blipper:

Firdell Blipper (model 210-5, model 210-7) was disappointing.
The Firdell Blipper is not an octahedral reflector, but it still uses the basic trihedral corner reflectors. Rather than eight corner reflectors oriented around a sphere as in the case of the octahedral reflector, the Firdell uses ten trihedral corner reflectors oriented approximately 36° to each other, and optimized for angles close to the horizontal. The theory is that by avoiding the regular geometry of the octahedron, the deep nulls can be avoided. Its a good theory, but the problem is that in order to fit the corner reflectors into a package of reasonable size, the individual reflectors must be made fairly small, with a radius of only 4" in the case of the popular 210-5. Since the performance of a trihedral reflector is proportional to the fourth power of its size, this is a serious loss. For example, a circular 6.25" trihedral element (such as the Davis Echomaster) will have an RCS 2.5 times (4 dB) greater than a 4" trihedral element such as the Firdell.

We tested two models of the Firdell Blipper, the popular 210-5, which measures 20" in height and 8½" in diameter, and weighs 3¾ pounds, and the slightly (10%) larger 210-7. Both are designed to be mounted vertically, either on the forward side of a sailboat mast, suspended vertically using a small halyard, or mounted vertically on a flat surface. It should be noted that mounting the Firdell (or any reflector) on the front of a mast will shadow it from the rear, making it ineffective over an angle that can be 90° or more.

Mobri:

Mobri reflectors are available in two diameters, neither of which performed well.
The cylindrical Mobri reflector is another variation on the trihedral theme, but in this case they are stacked in either a 2" or a 4" diameter cylinder. With the radar beam exactly at right angles, they act as a series of dihedral reflectors, but even small heel angles cause it to operate in a deep null with little reflection. The series of end plates that would form the third side of each trihedral are too small to be effective, even in X-band, and are operating too close to edge-on at small heel angles. The smaller 2" diameter unit suffers an additional problem in that the 1" radius of each dihedral reflector is less than a wavelength even at X-band. Both Mobri reflectors have provision for hanging the cylinder from top and bottom, or can be strapped to a wire or spar. The manufacturer suggests mounting one on each cap shroud above the spreaders, which would provide a reflection at two narrow angles of heel, rather than just one.

High Gain Rotation:

The High Gain Rotation is a plastic sphere with a gimbaled quadrahedral reflector inside. In this photo, it is shown cut open.
The High Gain Rotation is an 8" diameter plastic sphere with a gimbaled quadrahedral reflector inside. Unable to determine how the gimbaling was accomplished, we cut open the plastic shell after testing. It has two intersecting aluminum plates which are embedded in a combination float/ballast base, which in turn floats on what appeared to be water. This allows the reflector to remain vertical through 360° of pitch and roll. It does not allow the attitude of the reflector surface to be known while testing, however, since it is completely enclosed in the sphere and it is free to rotate. In addition, the effect of rapid boat movement is difficult to predict, since the period of oscillation of the reflector will be in and out of phase with the motion of the boat. Our test gave little insight into the workings of the High Gain Rotation, except that its performance at 0° and 20° of heel was very similar, which would be expected.

Cyclops:

The Cyclops 1 (the smallest of three models) has a sturdy masthead mount.
The Cyclops 1 is the smallest of the three Cyclops models, and has trihedral reflectors facing fore and aft and biconic reflectors facing athwartship. It designed to be masthead mounted, and has a provision for attaching masthead lights above it. It is a sealed plastic dome, with pointed ends fore and aft, and measures 13"L x 10.5"W x 7"H.

Non-Trihedral Reflectors

Lensref:

The Lensref is expensive, but because it works on the Luneburg lens principle, return of the X-band signals was very good. Unfortunately, performance drops off beyond about 18° heel. Gimbaling or damping roll would help a great deal.

The Lensref is a Luneburg lens device, and is the one significantly different reflector that was tested. The Lensref is an 8" diameter sphere with layers of plastic (frequently likened to the layers of an onion) which vary in their index of refraction. By focusing the radar energy to a reflective band around the "equator" of the lens, and then back along the same path to the source of the energy, a claimed 360° reflection is achieved. It has a 10 mm bolt at its top and bottom which can be mounted in an optional mast bracket, or bolted to the top or bottom of a vertical surface.

The angle of heel is limited, however, by the width of the metalized band that provides the actual reflection. Beyond about 18 degrees of heel the focused beam misses the metalized band completely, and the reflector quits working. Providing a wider reflective band would increase the range of heel angles, but at the expense of overall performance since more of the "front" surface (towards the radar beam) would be covered. Mounting the Lensref on a gimbal would significantly enhance its performance under sail.

Radar Flag:

The Radar Flag reflects satisfactorily at 0° heel when flat, but in the draped position in which it would be used on a staff, it is practically invisible, giving an average return of just 4 ducks.

The Radar Flag is a fabric U.S. flag measuring about 20" x 11.5". Sewn inside the fabric is a metallic cloth which has reflective properties. The flag is intended to be flown on a conventional staff at the stern of a boat, to be allowed to flap like a normal flag. Construction is nylon, with heavy sewing for reinforcement.

The Tests:
We were very fortunate in being able to secure the use of the radar testing facilities at SRI International in Menlo Park, California. The tests were conducted in SRI’s large anechoic chamber, which measures 20’ x 20’ x 40’. The target is centered near the far end of the chamber, on a radar-transparent pedestal that can be rotated through 360°, and the axis of rotation can be tilted up to 12°. A calibrated broad-band microwave transmitter/receiver is located in the wall at the opposite end of the chamber to accurately measure the reflected signal.

The walls, ceiling and floor are completely covered with semi-conductive, radar absorbing foam pyramids which absorb any stray radar signals and prevent any reflections back to the receiver, except those from the device under test. A retracting gangway extends through a door in one wall (also covered with foam pyramids) to provide access to the test pedestal.

The chamber is calibrated in terms of absolute RCS, and optimized to measure the very small radar returns from certain types of military aircraft. The background return is on the order of -60 dB (a millionth of a square meter). Calibration was checked before and after each days testing.

An HP minicomputer provided data logging and azimuth-elevation control of the test pedestal. Data was taken simultaneously at 3.05 GHz (S-band) and 9.41 GHz (X-band), recorded directly to disc and plotted to a laser printer. The data was subsequently converted to text files and transferred to floppy disc files for further analysis.

The first series of tests were performed during a two-day period, October 20-21, 1994. For each test, the gangway was extended, and the reflector being tested was secured to the test pedestal with non-reflecting tape and foam. The chamber was then closed, and the reflector was rotated at 1° increments through 360° while data was recorded. Each rotation required about 20 minutes.

Most of the reflectors were tested twice. A first test was done with the major axis perpendicular to the radar beam, corresponding to a reflector mounted upright on a vessel with no angle of heel, and viewed from every angle around the vessel. A second test was done with the reflector secured at an angle to the pedestal, corresponding to a reflector mounted upright on a heeled sailboat while a ship steams a circle around it. In this latter case the radar beam strikes the reflector at angles along an inclined plane, both above and below its equator.

The Results:
The results of selected individual tests are shown in graphical form as figures 4 to 8. Radar Cross Section in m2 is shown as polar plots for both X-band and S-band, indicating the strength of the reflected signal in that would be seen by a ship steaming in a circle around a reflector located in the center. For each plot the outer ring represents the minimum RCS threshold (2.5 m2 for X-band and 1.0 m2 for S-band), and the inner ring represents 1.0 du, one duck unit.

Using the criteria for minimum reflectance described above, Table 1 was prepared as a summary for all of the devices tested. For each radar band, the first column indicates the average Radar Cross Section (RCS) in square meters for the reflector, using the recommended RMS (Root Mean Square) average. The second column indicates the percentage of angles that were greater than the minimum threshold (2.5 m2 for X-band), an indicator of the "visibility" of the reflector, or the probability of being seen by a ship at an unknown horizontal angle. The third column indicates the largest angle that was less than this threshold, i.e. the angular width of the largest "blind spot". For S-band, a minimum threshold of 1.0 m2 was used, reflecting the 4 dB reduction in sea state return experienced at S-band, allowing a smaller return to be detected.

The table data are sorted first by X-band "visibility", the percentage of return greater than the threshold, then by average RCS for the reflectors with no return above the threshold.

Using this criteria, the Davis Echomaster was the clear winner, but showed the deep nulls associated with an octahedral reflector. The peaks were as high as 25 m2, but these peaks were too narrow to have any real significance. S-band performance was lower than X-band, but by less than the expected factor of 10.

The best overall performance for the Davis was not in the often-recommended "catch rain" position, but in the "double catch-rain" position, which has the advantage of very little degradation of performance with heel. The average RCS was 5.0 m2 upright, and 4.5 m2 heeled 20°. Visibility was not great, however, at less than 50%

The vertex-up position provided the best performance with a 0° heel angle, but quickly deteriorated as the reflector was heeled, and is not recommended.

The Lensref was a close second in this tabulation, with an average RCS of 2.4 m2 (see fig. 5). Interestingly, this is just a fraction below the somewhat arbitrary threshold of 2.5 m2. If we arbitrarily assign a threshold of 2.0 m2 instead of 2.5 m2, then the Lensref goes straight to the top of our chart, with virtually 100% of the return greater than the threshold (i.e. no gaps), an outstanding performance amongst this lot. Only three of the Lensref data samples came in at less than 2.0 m2, and then only by one or two hundredths, truly splitting ducks. Performance of the Lensref on S-band is pretty marginal at 0.4 m2 average RCS, due to its small size compared to the wavelength.

The limited angle of heel is a serious limitation for the Lensref for a sailing application, as angle of heel is often greater than the 18° limit of the device. Fitting a gimbal would solve this limitation neatly, but would need some engineering to avoid uncontrolled swinging, as the Lensref is not a lightweight device.

The poor performance of the Firdell Blipper was surprising, given its popularity and reputation. When measured at X-band with no heel, the 210-5 model fitted to most boats was only visible over 7% of the horizontal angles, and only a few peaks exceeded the 2.5 m2 threshold (see fig. 6). The average return was 1.4 m2, or 14 ducks, and the largest gap was over 90°. When heeled to 20°, the performance of the 210-5 deteriorated about 20%. The larger 210-7 model had a 20% higher average return of 1.7 m2. Measured at S-band, the 210-7 performed about 50% better than the smaller unit, and was "visible" for 26% of the angles compared to 8% (with a 0° angle of heel).

The Radar Flag reflector gained a high ranking in its flat configuration, spread out in a vertical plane. In this orientation it exhibited a very strong return perpendicular to the plane of the flag, but almost no return at other angles. It was "visible", with a return above the threshold, for only 9% of the angles. In a more typical "drooped" orientation, the Radar Flag was essentially invisible, with an average return of only 0.4 m2 (4 ducks), and not above the threshold at any angle.

 The Davis Emergency reflector is the last of the lot to provide a return above the threshold at any angle, but is far from its bigger brother. The other reflectors were generally limited by their small size.

The two Mobri reflectors performed as might be expected, and were essentially invisible. Only the larger 4" diameter (2" radius) device came anywhere near detectability, with an average return at a 0° angle of heel of just over 1 m2, with no deep nulls. On S-band, the average return was almost 0.5 m2, not enough to be detected, but better than most. When heeled, however, things fall apart and the return drops to a few duck units. The smaller Mobri is invisible under all conditions, and, with its minimal windage, might make a nice addition for the Stealth Bomber.

Target Pattern Maps:
Following this series of tests, the data was shared with some of the manufacturers. GEC Marconi, manufacturers of the Firdell Blipper, responded that to quantify reflectance in a single plane does not represent the best way of looking at reflector performance. Specifically, they asserted that, with respect to the Firdell Blipper, the large nulls that were observed in the horizontal plane would be small in the vertical direction, and that only a one or two degree change in elevation angle (i.e. heel) would move from a null to a peak. It was further asserted that presenting the data in the form of a three-dimensional Target Pattern Map was the only proper representation of reflector performance.

A Target Pattern Map (TPM) is a method of representing three-dimensional data on paper, where azimuth (horizontal) angles are shown on the horizontal X-axis, elevation data is shown on the vertical Y-axis, and the strength of the return is shown by color or gray-scale shading. Color is dramatic, but unfortunately expensive to reproduce.

A study of GEC Marconi’s published TPMs reveals two interesting anomalies. First, the comparison to a 12" octahedral shows peaks of barely 2 m2, while a 12" diameter spherical octahedral reflector (such as the Davis) would be expected to have a peak RCS associated with the axis of the "pocket" of each trihedral reflector of 8.3 m2. The answer to this mystery may lie in the formula for the return for a triangular trihedral. An octahedral made from 8½" square plates would form triangular trihedrals, and would measure 12" (i.e. a 6" radius) across the largest dimension. The theoretical peak return for this device would be 2.2 m2, which corresponds to the Bell/Lark TPM. So the comparison must be to a 12" triangular octahedral, not a spherical octahedral such as the Davis Echomaster.

The second problem with GEC Marconi’s data is with the characteristic of the peaks for the Firdell Blipper 210-7 shown in the TPM. A preponderance of peaks are shown greater than 3 m2, with a vertical interval from peak to peak of about 3°, the basis for GEC Marconi’s statements that the "gaps" are "only between 1 and 2 degrees wideiii". The vertical peaks and nulls are the result of interference effects between vertically spaced reflector elements, which will either add or cancel as the heel angle is changed. Both the size and the vertical spacing of the reported peaks, however, are consistent with a device much larger than the 210-7.

A second series of tests were performed on 26 March, 1995 to investigate these assertions. The computer-controlled target pedestal elevation control was used to take data at elevation angles from -12 to +2 degrees and azimuth angles over 180 degrees without remounting the reflector. That range of angles was felt to be representative, and was within the time available in the chamber. Data was collected for the Davis Echomaster and the Firdell Blipper 210-5.

Interestingly, it was not possible to reproduce the results reported in the Bell/Lark paper. The Davis Echomaster showed the expected pattern, with a broad reflectance peak corresponding to the axis of each trihedral corner reflector, and a sharp peak matching the orientation of each planar surface. The returns associated with each trihedral reflector peaked at just over 6 m2, compared to a theoretical peak of 9 m2. The average (RMS) return was 2.6 m2 for the Davis over the area tested, consistent with the earlier tests.

The Blipper results that we obtained were quite different from those that were reported in Bell/Lark. The areas of low reflectance extended much further in elevation than reported, with a vertical interval of 6 to 7° between peaks. The overall average RCS was 1.6 m2, only slightly higher than our previous more limited tests, and only a very few peaks exceeded 3 m2. The issue of the size of the gaps seems a bit moot given the small size of the peaks. Figures 8 and 9 show the TPM data that we obtained for the Davis Echomaster and the Firdell Blipper respectively. The 210-7 is approximately 10% larger than the 210-5 tested, and would be expected to show peaks about 40% higher but only 10% smaller vertically, not a significant difference.

While TPM’s are certainly a more comprehensive way of looking at reflector performance, for these types of devices a simple horizontal scan at two or three heel angles is more than sufficient to characterize the reflector. The additional complexity of a Target Pattern map is not justified for any of the reflectors tested here.

Conclusions:
The first conclusion is that there is no substitute for size when it comes to radar reflectors. The devices that offer smaller size and lower windage simply don’t work as well. With regard to the Firdell Blipper, it is a well packaged and clever device, but the models tested were not large enough to have much real value aboard a vessel. Larger versions would accomplish what GEC Marconi claims, but are not practical on small vessels.

The Davis Echomaster (in the "Double Catch Rain" position) and the Lensref performed the best of all of the devices tested. The Lensref has no nulls, which is a tremendous advantage in terms of being seen, but the overall reflectance is marginal. If a Lensref is fitted on a sailing vessel, it should be gimbaled or made adjustable. The Davis Echomaster had stronger peak reflectance, but also large holes, which means that a large target would not consistently be presented on a ship’s radar.

None of the reflectors would be more than marginally useful in offshore situations where only S-band were being used, except perhaps in calm sea conditions.

The marginal performance of radar reflectors in general does not mean that they should not be carried. On the contrary, anything that improves a vessels radar visibility is worthwhile, particularly short-handed vessels and those without radar themselves.

Beyond that, it needs to be again pointed out that the best defense where shipping is concerned is a good offense. A ship’s radar may only see a sailboat three or four miles away, but that same sailboat can typically see the ship 12 miles away by radar, and visually at least 8 miles away in clear weather. The small boat is both better equipped and more highly motivated to avoid the potential collision.

Kilde: www.ussailing.org

Selecting a radar reflector.

There are a few very important factors to be taken into account when selecting a radar reflector for an offshore cruising boat. Offshore being the operative word, because a different or better radar reflector may be required offshore because ships use two different types of radar depending on whether they are coastal or offshore, and also the weather at the time. When a ship is close to the shore it may use X-Band radar, and when it is deep-sea it may use S-Band radar. This raises a rather uncomfortable reality for cruising boats, in that most radar reflectors are good with X-Band radar but are not necessarily so good at reflecting S-Band. I am told that the new IMO regulations require good response in both bands, newer radar reflectors are about to be launched (Jan 2005), so keep an eye on new products.

There is another angle to this and that is simply that in good weather deep-sea ships often do not use radar at all. So we may even be putting too much trust in our radar reflectors. Having said that, we should do everything we can to minimise the risk of being run-down, and don't forget that in bad weather such as heavy rain, fog etc ships will be using their radar and it would be very nice if they were able to see us.

When cruising short handed, keeping a thorough 24 hour watch is very demanding and we may all be guilty of at least occasionally not being as thorough as we should especially when we are very tired. Don't forget that fibreglass and wooden boats are extremely poor at reflecting radar. Having a good quality reflector may help in being seen when our guard is down.

Unfortunately when choosing a radar reflector many people are using the wrong criteria to select a device. The worst of these is size, trying to get the smallest device to perform a task that is ideally performed by a large unit is a bit like complaining that a parachute is too big and heavy and we want a smaller one. The physical size of a radar reflector provides us with a bit of a conundrum, none of us really want a very large cumbersome and drag inducing device in our rigging but the dilemma is that for radar reflector to perform its function well it needs to be fairly large. Manufacturers' have come up with clever ways of trying to return a good echo from a small device. It is important to carefully study the figures for each design, check the test results where available and compare them with other designs. Most manufacturers will supply test data if requested.

An ideal radar reflector should reflect the radar signal through 360 degrees at large and varying angles of heel. When viewing the test figures of the various models available it will be seen that very few radar reflectors are actually capable of returning a signal through the full 360 degree spectrum especially when heeling, but even when they are not heeling! It will also be noticed that standards have been set by certain authorities that do not require 360 degree performance, which probably reflects on the difficulty on producing such a radar reflector.

It should also take into consideration that regulations may apply to yachts in certain countries regarding the size and type of radar reflector required, you should check the regulations for your home country.

There are several different technologies to choose from; there are the standard passive reflectors which themselves use different technologies and then there are some active systems which require a power supply to enhance and return an echo.

The choice of a radar reflector will depend on many factors such as the size of the vessel, regulations, personal preference and the size of the budget. Also new technologies are emerging and although the new type of reflectors look very good, we need to see some good independent tests as these new models hit the market. Some vessels may spend most of their time coastal cruising and others may plan to spend more time offshore which may affect the choice of radar reflector.

Kilde: www.onpassage.com

ANNEX 15 - Radar Reflectors

Regulation 19 para.2.1.7 requires radar reflectors to be carried, where practicable, by ships under 150 GT. For UK-flagged this includes pleasure vessels.

The following notes gives further guidance on the choice of a radar reflector for small vessels and supersede Merchant Shipping Notice M.1638.

1.) Reflectors meeting the standards laid down in British Standard BS 7380:1990 (ISO standard 8729: 1987) meet IMO performance standards*. Radar reflectors which were type tested and approved to the earlier DOT Marine Radar Reflector Specification, published in 1977, also comply with the IMO standards.

2.) An important parameter of a radar reflector is it's echoing area, or equivalent radar cross-section, as this determines the amount of the radar energy which is reflected back. Reflectors to the above standards have a maximum echoing area of at least 10 m² with a minimum echoing area of at least 2.5 m² over 240° of azimuth. Orientation of the reflector must follow manufacturers recommendations if it is to be effective.

3.) Regulation 19 takes account of the fact that reflectors built to the above standards are relatively large and may not be practical for fitting to smaller vessels. The Agency considers that fitting reflectors meeting IMO standards to vessels of 15m and above length should be practicable.

4.) Owners and operators of craft vessels of less than 15m in length should fit reflectors with the greatest echoing area practical. In all cases, the reflector should be mounted as high as possible for maximum detection range, following the manufacturer's instructions.

5.) It should be noted by Master and Operators of all vessels that even the 10 m_ reflectors referred to above will be difficult to detect in sea clutter on radar displays. Masters of all vessels are reminded that this should be taken in to account when setting lookouts and determining safe speed as required by Rules 5 and 6 of the International Regulations for the Prevention of Collisions at Sea.

6.) Electronic radar target enhancers are now marketed by some manufacturers. Radar enhancers can be considered as “other means” in the Regulation. These have a larger equivalent radar cross-section for a physically smaller size than radar reflectors and produce a response on a radar display, which is stronger and more consistent, but does not increase the apparent size of the target. Some navigation buoys are being fitted with electronic radar enhancers and seafarers should be aware this improves their detection range. Mariners should note that radar enhancers currently available do not operate in the radar “S” band.

7.) Owners and operators should note that under Regulation 18 equipment meeting the requirements of Regulation 19 must be type approved. However by virtue of Regulation 1.4, the Agency allows United Kingdom vessels which are too small to fit reflectors meeting the IMO standards to fit equipment suitable for the type and size of vessel.

Kilde: www.onpassage.com

Echomax V Blipper

The above test results were obtained at QinetiQ (DERA) in April/November 2001 and May 2002. Why choose EchoMax?

Radar Cross Section (RCS) explained
1. A sphere operates with a weak signal at all angles of incident
radiation.

2. A flat plate is an extremely efficient reflector but has a very sharp angle of response.

3. RCS may, for practical purposes, be defined as the cross section area of a conducting sphere of such a size that it would return an echo equal in strength to that of an equivalent flat plate oriented so as to be perpendicular to the direction of the incident radiation.

4. One metre squared is the cross section of a sphere radius 0.565 metre (R2xPi - 1 metre 2)

Collision Avoidance times

Radar Interference by Sea State & Precipitation
Mariners Handbook Fourth Edition 1973

Sea' is the name given to waves generated by wind blowing locally. A radar screen becomes cluttered when echoes from waves are received. Further clutter arises from precipitation (rain, snow, and fog).

Sea States :
Moderate : 1.25 to 2.5 metres
Rough : 2.5 to 4. metres
Very rough : 4 to 6 metres

Wave Clutter:
Echoes from upwind are greater than those from seas running down wind. Wave clutter does not extend beyond five nautical miles but large echoes arise as beam grazing angles increase.

Precipitation:
Rain
Light: 4mm/hour
Moderate: 10mm/hour
Heavy: 16mm/hour

Fog is caused by the cooling of air in contact with a surface at a temperature whereby it can no longer maintain, in an invisible state, the water vapour which is present in it. Condensation of the vapour produces minute, though visible, water droplets. Rain and snow are further examples of droplets which return radar clutter.
Transmitted power, to and from the target, is attenuated by precipitation on average by -5dB (-70% of reflected power) . Visibility will be a guide to assessing power RCS lost.
Note:- Precipitation and wave clutter may or may not occur together.
The mandatory collision frequency is 'X' band. Precipitation is penetrated better on 'S' band but its target response is 1/10th 'X' band. S Band does not overcome sea clutter but may be used from beyond three nautical miles to penetrate fog, rain, snow etc. A 10m2 target will return 0.001 m2 from ten nautical miles. Doubling the distance requires target area to be16 times greater .

F. J. Wylie 'The use of Radar at Sea'

Sea Force 4 - Wave height one metre.
Sea Force 6 - Wave height Two metres.
Approximate clutter from waves:

One of the most comprehensive tests of radar reflector performance was carried out in 1995 by West Marine www.ussailing/safety/ studies/radar.reflector. The conclusions were that the poor performance of the Firdell Blipper 210/5 and 210/7 were surprising given their popularity and reputation, and although well packaged and clever device, the models tested (i.e. 210-5 and 210-7) were not large enough to have much value aboard a vessel. Similar conclusions were drawn in in the Practical Boat Owner test published in a series of articles see issue 391 July 1999. “Having so many reflectors close together produces a polar plot made up of several spikes ‘a good response' separated by an equal number of deep interference troughs in which the reflection from one corner reflector cancels out the reflection from the other.” The EchoMax range is included in the 2003 West Marine catalogue, replacing the Firdell Blipper.

The above figures illustrate the 'TriLens' 5 inch diameter reflector, for which they claim 2 to 4M2-RCS, will be hidden in moderate clutter. They also state its performance is comparable to a 12 inch corner. Presumably they mean a 12 inch octahedral RCS-2.21M2. The RCS of a single twelve inch corner is 35M2.

Rosendal's web page compares their Mini-TriLens with a Mobri reflector found 'invisible' by West Marine. The performance of other reflectors, given below, was examined by QuinetiQ in the presence of independent observers.

** Latest literature we have seen accompanying Visiball states that "its computer generated surfaces ensures a consistent performance through 360 degrees and its special filling maximizes the reflection." Our tests at QinetiQ only gave a response of 1m2 for 195 degrees. The balance of 165 degrees being virtually zero response.

For Echomax performance see website.
Echomax EM 230 weighs approximately four pounds, and comprises six identical sectors each responding with peaks to 24M2, 10M2 - 70% az., 5M2 - 100% az. When the reflector is tilted, with yaw pitch and roll, 'glint' also ensures an all round performance is maintained.
So far as we can discover we are the only manufacturer who freely publishes independent test results. We also include with each product a certified polar diagram of it performance.

ISO 8729 extends to six pages and is available from HM Stationery Office, the pertinent performance paragraphs are:

5.1.1 The maximum echoing area of the radar reflector shall be at least 10m2

5.1.2 Its azimuthal polar diagrams shall be such that its response over a total angle of 240 deg. is not less that 2.5m2. The response shall not remain below this level over any single angle of more than 10 deg.

5.1.3 These requirements shall be assessed by reference to related azimuthal polar diagrams about the reflectors vertical axis and tilted from the vertical at angles not exceeding plus or minus 3 deg.

5.2 Reflecting pattern in vertical plane The performance of the reflector up to at least plus or minus 15 deg. from the horizontal shall be such that its response at any inclination remains above 0.625m2 over a total angle of at least 240deg.

(Paragraph 5.2 is commonly accepted as flawed and was based on the performance of the 18” Octahedral.)

RORC - Royal Ocean Racing Club - ORC - Offshore Racing Council, now part of ISAF .

These operate the Offshore Special Regulations which prescribe the equipment etc., to be carried on Ocean Racing Yachts. The relevant part is regulation 4.10 which reads:

4.10 Radar reflector A Radar reflector shall be provided. If the radar reflector is octahedral it must have a minimum diagonal measurement of 456mm (18”), or if not octahedral must have a documented RCS (radar cross-section) of not less that 10m2. The minimum effective height above water is 4.0 m (13ft). Compliance with ISO 8729 is strongly recommended as a minimum standard. In addition to (but not in place of) the above, an RTE (Radar Target Enhancer) is recommended.

SOLAS Chapter V Regulation 19 para 2.1.7 (effective 01/07/2002)
Regulation 19 para.2.1.7 requires radar reflectors to be carried, where practicable, by ships under 150 GT. For UK-flagged this includes pleasure vessels.

The following notes gives further guidance on the choice of a radar reflector for small vessels and supersede Merchant Shipping Notice M.1638.

1.) Reflectors meeting the standards laid down in British Standard BS 7380:1990 (ISO standard 8729: 1987) meet IMO performance standards*. Radar reflectors which were type tested and approved to the earlier DOT Marine Radar Reflector Specification, published in 1977, also comply with the IMO standards.

2.) An important parameter of a radar reflector is it's echoing area, or equivalent radar cross-section, as this determines the amount of the radar energy which is reflected back. Reflectors to the above standards have a maximum echoing area of at least 10 m² with a minimum echoing area of at least 2.5 m² over 240° of azimuth. Orientation of the reflector must follow manufacturers recommendations if it is to be effective.

3.) Regulation 19 takes account of the fact that reflectors built to the above standards are relatively large and may not be practical for fitting to smaller vessels. The Agency considers that fitting reflectors meeting IMO standards to vessels of 15m and above length should be practicable.

4.) Owners and operators of craft vessels of less than 15m in length should fit reflectors with the greatest echoing area practical. In all cases, the reflector should be mounted as high as possible for maximum detection range, following the manufacturer's instructions.

5.) It should be noted by Master and Operators of all vessels that even the 10 m2 reflectors referred to above will be difficult to detect in sea clutter on radar displays. Masters of all vessels are reminded that this should be taken in to account when setting lookouts and determining safe speed as required by Rules 5 and 6 of the International Regulations for the Prevention of Collisions at Sea.

6.) Electronic radar target enhancers are now marketed by some manufacturers. Radar enhancers can be considered as “other means” in the Regulation. These have a larger equivalent radar cross-section for a physically smaller size than radar reflectors and produce a response on a radar display, which is stronger and more consistent, but does not increase the apparent size of the target. Some navigation buoys are being fitted with electronic radar enhancers and seafarers should be aware this improves their detection range. Mariners should note that radar enhancers currently available do not operate in the radar “S” band.

7.) Owners and operators should note that under Regulation 18 equipment meeting the requirements of Regulation 19 must be type approved. However by virtue of Regulation 1.4, the Agency allows United Kingdom vessels which are too small to fit reflectors meeting the IMO standards to fit equipment suitable for the type and size of vessel.

Paragraph 5 above confirms our view that radar reflectors with an RCS of 10m2 or less are of little or no use in real sea conditions.

The following reflectors do NOT meet ISO 8729 and therefore do not satisfy SOLAS Chapter V, RORC or ORC requirements:

- Mini-Trilens, Trilens 5.25 Inch lens, Cyclops I & II, Mobri 50mm/100mm, Blipper 210-5/210-7, Pains Wessex SC4, all Octahedral based reflectors under 18 Inches Diameter, including Davis Octahedral, VisiBall.

The EchoMax Midi meets RORC and ORC requirement of 10m2 peak but fails ISO 8729 by 2 degrees in each sector.

ECHOMAX COMMERCIAL the professionals choice

Andrew Ridley, Conservancy Operations Manager of Tees & Hartlepool Port Authority Limited first discovered Echomax at Seawork 2001. He was so impressed with their QinetiQ test results and a series of on-site tests that he insisted Echomax arrays were fitted into all the new Eason Marine buoys which he was in the process of ordering. Eason Marine, probably one of the largest suppliers of navigation buoys in the U.K, modified their top marks and cases to suit the EM230/305 arrays. Since they have been laid he has received unsolicited compliments regarding both the radar response and visibility of these units. It was confirmed that the radar response from the Eason Polyethylene 2.4m maintenance free buoy fitted with Echomax arrays was at least equal to if not better than 3m steel buoys fitted with 32" octahedral reflectors. (It is probably only the sheer mass of the metal buoys which has enabled the octahedral reflectors to be fitted for so long.)

Mr. Ridley was also receiving complaints from the Harbourmaster when another commercially available reflector fitted to a GRP pilot boat built in 2000 provided inconsistent response resulting in loss of the traces on the VTS radar. As there was insufficient mast height to fix a standard EM305, a customised 305 was fitted into a GRP roof box/mast support. Although additional height above sea level would have been beneficial it was reported that the EM305 gives a much improved response signal. An EM305 fitted to a fixed wooden finger jetty has enhanced the radar signal through all tidal states and avoids the reflection being lost when a combination of high tide and sea clutter would otherwise have caused problems to approaching vessels.

New white and orange Echomax 230's have now been fitted to all seven vessels in Tees & Hartlepool Port Authority's pilot and workboat fleet to comply with SOLAS Chapter V regulation 19

Sea Trip on Trinity House vessel Vectis to view Barrow - Black Deep 7 fitted with ECHOMAX EM 305 Top marks

Echomax EM305 Top Mark as fitted to Barrow and Black Deep 7 Trinity House buoys

After just one and a half hours steaming from Harwich on the Trinity House Vessel Vectis, Black Deep 7, green channel buoy was picked up 6.84 miles ahead. The Skipper of Vectis remarked that Echomax radar response was brighter than 11m Sunk Tower beacon with top mark. Half a mile closer we picked up the response of corresponding Trinity House Red Inner Fishers buoy. The very calm sea conditions did not demonstrate the superior response given by Echomax at heel.
All parties are aware that the Decca scanner is set very low on the Vectis mast and had this been higher all objects may well have been picked up earlier.

Kilde: www.sailgb.com  ( This document was provided by Echomax )

Be Seen or be Sorry! 

All vessels and navigation marks need to be seen by radar at all times, in broad daylight as well as in darkness, rain and poor visibility. The more so, Captain A.P.S. Lark, Chairman of the Sea Safety Group (UK) wrote in the Nautical Institute of London's consul tative paper Radar Detectability and Collision Risk Uanuary 1994), because "The combination of sophisticated electronics and low crew numbers (at all levels of commercial shipping) means that radar has now become the prime means of keeping a look-out."
    What is radar, anyway? Very simply it is a radio beam shone out from a radar transmitter as it goes round and round - much as a lighthouse beam shines out from a lighthouse. Ideally, when our radar target is struck by part of the radar beam, it should "bounce" as much of that radar as possible back along a reciprocal course, to be gathered in by the "scoop" on the transmitter and fed down to the radar screen where it appears as a "blip".
    Nothing could be simpler - but this is where snags creep in. Ignoring the scientific reservations (which don't make much difference to the end result), a beam of radio or light bounces off a reflecting surface it an angle like that of a snooker ball bouncing off the cush. So the first thing to remember is that a lump of metal or any- thing else standing in the path of a radar beam does not guarantee a reflection back to the source of transmission.
    TV and radio tell all. "The search will be resumed at day- break" or "when visibility improves" mean that all the sophisticated radar equipment of air and surface searchers cannot detect a vessel because its radar return is poor or even non-existent.
   The reasons are many and varied. Most radar energy striking a boat is deflected away uselessly into space, rather than reflected to the transmitter, while some materials do reflect as well as others. Steel and aluminium, for example, are good reflectors, but on boats are mostlydeflectors.
    Some materials do not reflect at all. Of those to be found in any quantity on yachts and power-boats, materials such as GRP and some other plastics, rubber compounds and wood are to a greater or lesser degree transparent or absorbent to radiation and do not give a good radar return. The materials and low profiles of yacht and power-boat hulls are usual ly such that any echo is unpredictable, and while a thankfully diminishing number of brave souls still put their trust in upperworks, out board engines, wet sails, metal masts, rigging and deck fittings, any echoes from these will be small and erratic and of little if any value to a radar.
    Radar can only be as good as the 'ability' of a target to return its signals. To be of real value, radar needs "co- operative" targets. A vessel or mark is not of itself a co-operative target. To be so it needs to carry at all times an effective radar reflector, one which in sea-going conditions gives a reliable return to any number of radars which may be scanning it simultaneously from any point of the 360° horizon (azimuth) and, because of the continuous 3-dimensional movement of all things at sea, through at least plus and minus 15" of heel. Bear that 360° x 15° figure in mind.
    There have been a few optimistic (to put it kindly) claims for the reflective capabilities of kitchen foil, "radar flags", reflective tapes and balloons, etc, but if there are any worthwhile returns they are highly erratic in direction and we should forget them. In radar reflection Big is Beautiful. No radar reflector can return more radar than strikes it. Nor can it "intensify" radar before it is returned. The smaller the reflector, the less the chance of an identifiable and, more important, consistent echo on a searching radar's screen.
    The "radar capture area" of a reflec- tor, roughly width times height, is critical, because the amount of radar returned to a transmitter is proportional to the square of the radar capture area. Reduce the width and the height of the reflector by 2 and the area is reduced to a quarter, but the "Equivalent Echoing Area" or "Radar Cross Section" (see below) is reduced by a factor of 16, that is by 2 x 2 x 2 x 2. Double the size of the reflec- tor and you multiply by 16.
    The performance of any type of radar reflector is best mea- sured on a calibrated radar test range. A radar reflector is totally passive. It cannot "work" one minute and not the next, so beware reports of "comparisons" or "tests" of radar reflectors carried out at or over sea, where accurate calibration and observation is impossible and where radar, not the reflector, is subject to unmeasurable environmental influences which can literally ;change by the second.
    Again best to simplify. In systematic range testing a radar reflector is set on a turn-table in its recommended hoisting position. Radar is fired at it and the "quantity" of radar it returns is recorded. It is then turned, typically through 1 degree, and the radar return is measured again. The process is repeated until 360 separate measurements have been made. The reflector is then heeled, again typically 1 degree, and the procedure is repeated until, say, 11,160 individual measurements are recorded, representing performance at 1 degree steps through 360° x ± 15° heel.
    These results are usually expressed in "square metres" (m²) of Radar Cross Section (RCS) or equivalent echoing area. These are not measurements of physical size or area,but figures achieved by comparing the quantity of radar returned from the target reflector with the equivaleny return frim a radar reflective perfect sphere. Using the familiar pi x r² formula, a radar reflector's return of 2.5m² is thus equivalent to the radar return of a sphere 1.79 metres in diameter, 2.5m² in cross section area.
    So what should we look for in a radar reflector, regardless of type? That in materials and construction it should be sturdy enough to stand up to the inevitable battering of sea, wind and gear in a marine environment is obvious. For yachts particularly it should be lightweight and have low wind resistance. The materials from which it is manufactured should not through construction or damage be vulnerable to moisture absorption, because moisture is yet another radar deflector and absorber. Any casing material should be as near radar transparent as possible (avoid GRP, for example, only 3mm of which will absorb some 50% of the radar fired at it).
    Performance, largely through the continued absence of an internationally acceptable standard and consequent ambivalent claims by some manufacturers, is more difficult. Here are a few don'ts and do's. Beware ,the sales brochure that quotes a "peak" of maximum reflection as if it is all-round return, for example "a 10m² RCS" or "a 20m² RCS" reflector. That l0m² might be a single peak a degree or so in diameter, alongside a wide sector where there is no useful return at all.
    Beware the "single horizontal polar diagrams" that show performance at one or two selected elevations (just one turn each time through 360 degrees on that turn-table) as if they indicated performance through 360° x ± 150°. The gradually disappearing "octahedral" radar reflector, designed in the late 1930s and critically analysed in performance in the much- quoted but unfortunately "Classified" British Admiralty Monograph No. 833 of 1948, is an example of one such.
    The familiar, (in the Radar business) "single horizontal polar diagram" of the big 18"-diameter version shows the reflector at its best Level of perfor- mance,· in the recommended "catchrain" hoisting position. (Question: How do you keep it in the catchrain position relative to the scanning radar(s)?) Even here it shows six 20° -wide "gaps" in coverage. But all things move at sea, and as the carrying vessel and/or octahedral heels or pitches the performance deteriorates until at only 15° of heel there are three azimuthal gaps, each of 60°. The polar diagram which illustrates this rarely if ever appears in sales literature. (Contact Firdell Radar Reflectors - for copies of this.) Tel: 0208 420 3306
    The famous London chandler Captain O. M. Watts once described relying on an octahedral as like driving a car with brakes that would fail 30% of the time on the flat and 50% of the time on a 15° slope! And that was an 18" octahedral. The 12"-diameter version, which is more often seen on "small" boats, has an even worse perfor- mance, as you can see by applying the "radar capture area'' rule to its one-third reduction in size.
    The buyer should beware and always look for statements of full performance through all of a 360° x ±15° band at least. Some radar reflector manufacturers back their claims with Target Pattern Maps or TPMs, in which those 11,160 or similar measurements or analysed calculations are digitally presented in colour and graphically show a reflector's true all- round performance in the form of a 3D geographical contour map, with all its peaks and troughs of radar return. Better, because it is good to be sure that 15" doesn't mark some sort of cut-off pointsome manufacturers extend their TPMs to cover ±20° or +30° ofheel. Go for the largest reflector your boat can sensibly carry, given a minimum average return of about 2.5m² across that 360° x ±15° band. First because 2.5m² is generally accepted in the radar business as the "threshold" of radar detectability; second because isolated peaks of return are not as important to detection as consistency, particularly now that modern radar systems like ARPA (Automatic Radar Plotting Aid) can reject or eliminate erratic echoes from their screens. To that end, "gaps" in the band where there is no return above 2.5m² should not be more than about 5° x 5° in latitude and longitude, what is described as a "Window of Radar Observation". In the terminology of radar reflection, gaps bigger than this are called "Critical Gaps", and are unacceptable in a radar reflector.
    An effective radar reflector is one that gives a consistent, "seeable" echo through 360° of azimuth and at least plus and minus 15° of heel, without "critical gaps".
   In choosing a reflector it is sensible to select one with these performance -criteria demonstrated in a TPM, and to assume for safety's sake that your vessel alone does not give a radar return of sufficient quantity or consistency to influence a scanning radar. In that way, reflectivity from your vessel, if any, will be a welcome bonus.
    And with even the best radar reflector it is still good seamanship to maintain a visual look-out.

Kilde: www.cruising.org.uk  (Page created 20 September 2000)

Avoiding stealth.

Equipping your boat with radar and learning how to operate it properly is only part of the collision-avoidance story. The other part involves making sure that other vessels can pick you up on their radar. Anyone who owns a wood or fiberglass sailboat has achieved, at no extra cost, what the government has spent millions of tax dollars trying to achieve: virtual invisibility on radar. Image Credit: Courtesy North Seas Navigator, Inc.
2407

The solution is some sort of radar reflector. These run the gamut from simple and inexpensive to sophisticated and expensive. Buying one of these devices is a logical step toward increasing your radar visibility.

Radar is an echo-imaging device. A brief-duration radio frequency energy pulse is transmitted from a highly directional antenna, after which the set's receiver, using the same antenna, attempts to detect the very minute return signal that may reflect from various targets. No reflection, no information.

The power of the typical marine radar, often thousands of watts for even a small set, may sound impressive, especially when compared with the 25 watts produced by a fixed-mount VHF radio, or the one to five watts typical for a handheld VHF radio. However, this apparently impressive number is peak, not average, power, which is typically no more than a few watts. High peak power is necessary to provide reasonable assurance of target detection. The strength of the transmitted energy pulse is rapidly diminished by the distance over which it must travel. The transmitted radar energy does not behave like a highly focused, non-dispersing laser beam. In order to ensure target detection when the vessel rolls or pitches, it must be spread over a rather wide vertical angle, usually ±12°. Horizontal beam angles must be narrow in order to permit separation of closely spaced targets.

Typical horizontal dispersion angles for slotted waveguide antennas are 5.7° for a 16-inch antenna, 4° for a 21-inch antenna and 2.4° for a 42-inch antenna. The longer the antenna, the narrower the horizontal beam angle. (Radar units with long antennas, often more than 10 feet long, are commonly used on tugboats on inland rivers. Although antennas of this length are associated with sets having maximum range capability of more than 70 miles, their use on the rivers is dictated by their ability to provide excellent target discrimination. They can clearly show closely located targets as separate objects, while a shorter antenna, with its wider beam angle, would show the targets as one return.)

Diminished returns

By the time the radar's signal reaches a distant target the energy is spread over a wide area. Under the best of circumstances, only a small fraction of the radiated energy hits a target's surface. Of this, an even smaller portion will be reflected back toward the radar that originated the signal. The strength of this reflected energy will be further diminished by the distance it must travel back to the receiver. In addition, except under ideal conditions, a significant amount of both the incident and reflected energy may be absorbed by Earth's atmosphere. Heavy rain can totally blind most marine X-band radars, even at quite short distances.

Today's radar transmitters, antennas, receivers, and information processing circuits are truly excellent; however, without a decent return signal they are worthless. In order to gain the safety that comes from being visible on a vessel's radar we have to lend a helping hand. Our vessel needs to be a good reflector of radar energy. Marine radar sets operate on either X-band, centered at 9.41 GHz (9,400 MHz) with a wavelength of 3.2 cm (1.25 inches), or S-band, centered at 3.05 GHz, with a wavelength of 10 cm (3.9 inches).

Small-craft radar is primarily X-band, while large ships are fitted with both X- and S-band sets. The lower-frequency S-band offers advantages both in its ability to penetrate rain and in the reduction of confusing echo effects from the ocean's surface. The advantages of the lower-frequency radar are in part offset by its relatively poorer ability to detect and display small targets and to separate closely spaced targets. Ships typically rely primarily upon their S-band equipment when in the open sea, using X-band radar when in coastal areas. This fact can be important for those who voyage on open waters and wish to be as visible as possible on both X- and S-band radars. Regardless of the type of radar reflector used, it will be significantly less effective for S-band radar.

A radar reflector must be made of a material that is opaque at the frequency of the radar. Metal, even in the form of thin foil, is ideal for this use. In order to achieve satisfactory performance, the dimensions of a reflector must be a substantial multiple of the wavelength of the incoming radar energy.

Although pleasure craft are not governed by the requirements of the International Maritime Organization (IMO) or its referenced standards, IMO guidelines can be useful for determining desirable minimum performance for large vessels. The ISO standard for ship's radar reflectors requires a 10-square-meter effective equivalent reflecting area (radar cross-section, RCS) for an X-band radar. (A 10-square-meter RCS is the equivalent of a theoretical sphere with a diameter of 1.78 meters or 5.8 feet.) The specification requires that, when the reflector is within ±3° of horizontal, this reflecting capability must exist over at least 240° in azimuth, with no degradation greater than 6 db over an azimuth angle in excess of 10°. When the reflector is tested between ±15° from horizontal, the maximum degradation may not exceed 12 db. (Each decrease of 3 db is equivalent to a 50% decrease in reflecting area.) A radar reflector of a size practical for use on a small boat cannot provide a 10-square-meter RCS. From a practical standpoint, it is generally agreed that the minimum effective X-band reflector for small craft should be equivalent to a sphere whose projected area is 2.5 square meters (26.9 square feet). This radar cross-section will perform very poorly with S-band radar.

Reflectors dependent on shape

A sphere or a flat plate is a poor choice as a practical radar reflector. The sphere will reflect radar energy from any direction equally well or, perhaps more to the point, equally poorly. Depending on the relative angle of the arriving radar energy, a flat plate will work either very well or not at all. In this regard, the reflecting characteristic is like that of a mirror illuminated by visible light. Since there is no way of knowing at what angle the radar energy will arrive, using a flat plate reflector is pointless. An effective radar reflector must be able to present a significant RCS regardless of the direction from which it is illuminated by a searching radar signal.

The familiar, highly reflective highway lane markers, often called cat's eyes, are a type of corner reflector. The highly reflective coatings used on road signs are often made of miniature glass beads that act as corner reflectors. If three mirrors are placed at right angles to one another, a beam of light arriving from anywhere within a reasonably wide range of angles of incidence will be reflected back to the source of the light. Three metallic surfaces, placed at 90° to one another, can reflect radar energy, just as mirrors reflect light. The efficiency of the reflector will vary with the shape of each of the three plates. While square plates will be the most efficient, they will interfere with energy arriving at highly oblique angles and limit the effective angular range of the reflector. Plates in the form of quarter circles offer improved angular acceptance at the expense of somewhat less reflection efficiency. Plates in the form of triangles allow the widest possible angular performance, although with the least reflection efficiency of the three possible choices. Trade-offs are a part of technology.

Corner reflectors have some inherent problems. The performance of any corner reflector is close to zero when the incident energy arrives in-plane with any of the reflecting surfaces. An effective reflector must minimize the chance that incident energy from a searching radar falls on the reflector at a poor angle. If a vessel always sailed on a precisely even keel, with no angular motion about the roll, pitch, or yaw axis, designing and installing an effective radar reflector would be simple. Three corner reflectors, stacked one above the other, with each rotated 30° about the vertical axis, would provide an effective reflector for a radar signal arriving from any azimuth angle. Unfortunately, for both radar reflectors and those susceptible to seasickness, boats are rarely still.

One answer to the need for satisfactory reflector operation over a wide and ever-changing range of incident angles is the use of multiple corner reflectors, typified by the familiar radar reflectors. When tested in a laboratory, these devices will exhibit, as a function of incident angles, variations of as much as 20:1 in their radar cross-section. At angles at which the reflector is good, it can be very, very good and, to continue along the nursery rhyme theme, when they are bad, it is very, very bad. In addition to the variations inherent in the basic structure of the corner reflector, small angular variations from the desired 90° interplane relationship of the reflecting surfaces can reduce effectiveness. Corrosion may also have a negative effect on reflection capability. On the basis of lab tests in which the reflector is fixed in relation to the radar beam, tetrahedron-shaped reflectors tend to perform poorly overall.

Reflector never still

In the real world, however, the radar reflector is virtually never still. It is constantly moving about, presenting ever-changing attitudes to the incoming radar energy. The effect of vessel motion is usually amplified by the fact that these reflectors are usually hung in the rigging. Although the reflector's motion may at first seem a disadvantage, it may work to the benefit of radar visibility. As the reflector moves about, a poor incident angle may become more advantageous. This equation works both ways, changing what was a good reflection situation into an ineffective one. Since the advantageous incident angles of the multiple-corner reflector type outweigh the disadvantageous incident angles, the end result is positive.

We may be able to capitalize on the highly variable performance of simple corner reflectors by using more than one on a boat. Without undertaking a rigorous mathematical analysis, it seems reasonable to assume that, when two similar reflectors are non-rigidly mounted in a boat's rigging, the chance of both being in either the best or worst angular orientation to radar illumination is slight. Perhaps, while one is at its best angle, the other is at its worst, or possibly both are at some intermediate position. Given the relatively low cost of such reflectors, this may be a reasonable approach to solving the stealth avoidance problem.

There are radar reflectors that use technology other than the corner reflector. Some use dielectric lenses to reflect incident radar energy. Others use specialized dielectric lens configurations, such as a Luneburg Lens. When tested in a laboratory such devices perform very well, often far exceeding the reflection efficiency of a corner reflector of similar dimensions.

The Lensref uses a Luneburg Lens construction in which multiple layers of dielectric material act in a manner similar to a reflecting optical lens. The layers of dielectric material concentrate incident radar energy at a single reflective surface. The concentrated energy is then redirected back to the source via the same dielectric layers. Since the construction of this type of reflector is constant throughout 360° of azimuth, the performance of this type of reflector can be much more consistent than a corner reflector. Performance beyond a definite range of heeling angles is limited by the compromise necessary in the construction of the lens. Increasing the working vertical angle of the lens reduces the overall effectiveness of the device. Published data indicates that this reflector operates reasonably well up to about 18° of heel. Properly mounted in the rigging, this device should be at an acceptable angle much of the time. Its small size, eight inches in diameter, can be an advantage on many boats.

Corner reflectors and dielectric lenses

The Cyclops 3 reflector is composed of two trihedral corner reflectors, one facing forward, the other aft, combined with two dielectric lens reflectors facing port and starboard. The reflectors are housed in a plastic enclosure specified for masthead mounting. The corner reflector placement, facing fore and aft, recognizes that motion about the pitch axis is generally less than motion about the roll axis.

Test data collected at the British Defense Research Agency laboratory shows outstanding performance within a range of ±3° from the horizontal. As long as the angle from the horizontal is within this ±3° range, and with the Cyclops 3 mounted on a masthead or on top of some other structure, the resulting radar return should be very satisfactory. (As with all reflectors, shadowing of the reflector, caused by placing it close to a metal mast, will create a range of azimuth angles over which the reflector's performance will be seriously degraded.) Performance falls off as the angle of heel increases, but this happens with most reflectors. However, up to ±15° of heel, the azimuth angles over which performance was degraded were reasonably small. These areas of poorest performance were concentrated at ±40° of heel from directly ahead and dead aft.

The Cyclops 3, shaped like a squashed football, measures almost 18 inches in length, 17 inches in width and nearly 10 inches high. It weighs 18.5 pounds. Installation on the masthead may create some challenges for the installation of VHF antennas and wind systems. This device is also available in two smaller sizes, the Cyclops 1 and 2. The reflective value of these smaller units is not covered by the test data supplied by the manufacturer.

As with all problem solutions, there is a cost-benefit relationship associated with radar reflectors. Basic corner reflectors are available at prices from about $20 to $60. The Lensref is typically priced at about $475. The Cyclops 3 is $495.

There is an alternative to being a passive radar target. An active device can be fitted to a boat that will, when illuminated by radar energy, amplify the incoming radar signal and re-transmit it, enhancing the signal returned to the searching radar. The Ocean Sentry, Radar Target Enhancer, made in the U.K. by McMurdo Ltd. and marketed by Pains-Wessex, can substantially improve the radar signature of any vessel. Although somewhat similar to the operation of a radar beacon (racon) or the radar transponders used in aircraft, this device radiates very little power, typically 0.8 watts. However, even this small amount of energy can significantly improve the radar visibility of a target when compared with the few microwatts of a reflected radar signal. The Enhancer is composed of an X-band receiver (it does not operate on S-band) and a microwave amplifier housed in a 2.5-inch-diameter, 20.5-inch-long, 2.4-pound cylinder. In the absence of an incoming radar signal only the receiver is powered up, drawing 0.035 amps from the 12-volt bus. When a signal is detected the amplifier operates, enhancing the radar return. Power consumption with the amplifier on totals 0.250 amps. Average power consumption is minimal. Coverage is constant through 360° in azimuth.

McMurdo Ltd. claims normal operation for Ocean Sentry for angles up to 12° from vertical. The manufacturer specifies that the device is useful at ranges from 0.5 to 12 nautical miles. The specified minimum radio frequency gain is 55 db; typical gain is 58 db. Referenced to the radar cross-section of the theoretical sphere mentioned above, these gain specifications yield 25- and 50-square-meter reflecting areas (RCS). The Ocean Sentry is best installed at the masthead or on a separate antenna mast where it will not be shadowed by metallic structures. A small control box is mounted near the helm and provides the power switch, a self-test switch and a radar-nearby audible alarm. List price for the device is $1,995, with a street price on the order of $1,300.

Being seen by another vessel's radar is surely worthwhile. This assumes, of course, that someone on the other vessel is looking at his or her radar. In addition to being a cooperative target, it is clearly worthwhile to be an active participant in collision avoidance. Small but highly effective radar sets, many with waterproof displays, are available for less than $2,000.

Combined with one or more radar reflectors, a radar set can take much of the anxiety out of navigation in areas with limited visibility.

Kilde: www.oceannavigator.com - Chuck Husick ( Juli / august 1998 )

Omkring Aktiv Radar reflektor.

Aktiv Radar reflektor, Model SEA-ME

Good eNewsletter from Ocean Navigator in which the author describes a near collision situation and how he responded to it, using both technology and common sense... He points out how one device, an active radar reflector, could have been very helpful.
"Sea-me is an active radar reflector. When struck by a radar beam, Sea-me amplifies and reflects the amplified signal back to the sending unit. The amplified signal makes the target appear much larger. The reflected signal has an average radar cross section of 34 square meters, and I know vessels using this device often appear like a ship on radar. This device also has a radar detector, which alerts you to an active radar system in the area (similar to the CARD, though CARD does have a directional display)."

Kilde: www.panbo.com

Sea-me radar target enhancer, produced by a small company of the same name, sells in the same $500-to-$600 range as the Cyclops 3, but looks and works quite differently. Proprietor Peter Munro was an early-retired aerospace engineer when a heart-stopping 150-foot close call with a coaster off the island of Jersey in fog “inspired” him. He chose an “active” technique, in which a receiver listens for incoming radar pulses, amplifies them, and sends them back out. The technology is not new, but Munro’s goal was to improve the level of performance versus size and price. He feels he has achieved this goal with the Sea-me.
The Sea-me has been tested at 63-square-meter maximum RCS with no point showing less than 10 square meters, easily exceeding the IMO’s present requirements for passive reflectors. Real world users quoted on Sea-me’s Web site report noticeable effects, like being seen at 12 miles even in wave-cluttered seas. Another feature is the red “activate” light on its control box, which will provide a head’s up if traffic enters a radar-quiet part of the ocean. The Sea-me is not perfect. Though installing its 18"x2" antenna wand on a powerboat should be trivial, I’d be inclined to paint over its rather emphatic branding beforehand. More importantly, Sea-me only works with X-band radar. X-band is far and away the most common type of radar, and all SOLAS ships are required to carry it, but many also carry an S-band set, which they tend to use more in rainy conditions.
So, should you go out and buy a high-powered radar reflector? Not necessarily, but it’s certainly prudent to check out your own vessel’s inherent RCS. The ideal is to go out with a friend who also has radar and target each other at different ranges and headings. Remember that performance may be considerably diminished in rough seas. Ultimately, it’s up to you how big you want to look, but bear in mind that the IMO is concerned enough that it may soon triple its already challenging RCS minimums. Very few yachts are SOLAS vessels, but we all share the same oceans.
It’s also worth considering that for some boaters out there you might be the scary vessel that doesn’t see them. Study the RCS of any “small” boats you come upon when the visibility is good; excercise may teach you how to get the optimum target images from your set, and what might be missing when the fog does roll in.
On a final note, I’d like to issue a disclaimer. It seems I’ve written a string of columns concerning gloomy safety issues, but I certainly do not want to scare anyone away from boating. Being offshore, even at night, even in fog, can be beautiful and memorable and safe. Statistically, messing around in canoes or PWCs is more dangerous. But what the statistics don’t record are yachtsmen’s anxiety levels; inexperience, weather, and poor equipment can all conspire to suck the pleasure out of pleasure boating. If my cautionary columns help anyone increase their knowledge and confidence at the helm, that’s a good thing.

Kilde: www.powerandmotoryacht.com

Oktober 08   www.sejl.de